The present invention relates to image reproduction and, more particularly, to image data processing for bilevel printers or display devices which processing is equally applicable to text, continuous tone and halftone images.
Bilevel printing devices are characterized by having only two gray levels, e.g., black and white, for fixed sized spots on a prearranged geometric pattern. For bilevel printing devices, image data generated by scanning a document to be duplicated or displayed is processed generally dependent upon the type of data, i.e., text, continuous tone or halftone.
For the reproduction of continuous tone images, a number of processes have been developed based on pseudo halftone concepts which mimic the traditional halftone printing process wherein a variety of image gray levels are achieved through dot size variations. Since in these pseudo halftone processes each halftone spot is made up of a matrix of binary picture elements commonly referred to as pixels or pels, the effective resolution of the reproduction system is correspondingly reduced.
There are other processes which do not mimic the halftone process, but instead utilize localized information in deciding whether a particular spot should be printed or not. Error diffusion is one such technique having good detail rendition capabilities; however, it can also generate unwanted artifacts.
For text data, processes based on simple thresholding generally result in excellent reproduction. Such techniques include inherent enhancement features which sharpen the edges of line strokes. The same features, however, destroy any ability that the thresholding techniques might have in gray level distinction. Accordingly, text of different gray levels are reproduced with uniform density when a simple thresholding process is utilized.
Halftone originals present a different challenge to image processing. On a microscopic scale, halftone reproduction requirements are similar to those for text data. On a macroscopic scale, however, it is essential that the gray levels be duplicated without also introducing such artifacts as Moire patterns. Techniques for treating halftone originals include that of constructing a continuous tone approximation to the halftone original and then applying methods that are suitable for continuous tone images.
In many applications, there is a need to treat all three types of image data since all three types of images may appear either in a single document or in separate documents to be reproduced or displayed. One technique in the prior art is to include individualized data processes for each of the types of data with the detection of local micoscopic characteristics being used to determine which process to apply to a given document or a given portion of a document. However, switching between individualized processes for the different types of image data is not failsafe and, if an inappropriate process is applied to a portion of image data, the results are far from satisfactory.
It is, thus, apparent that the need exists for a single, universal data processing method which can be applied to data representative of any one of the image types, i.e., text, continuous tone or halftone.